(1) Field of the Invention
The present invention relates to a switching system having a selecting function of two-way lines, and particularly to a switching system that can reduce, irrespective of the structure of an opposite office, the possibility of simultaneous hunting of a single line by two offices.
Each two-way line can be hunted by a primary office or by an opposite office.
In actual use, the two offices may simultaneously try to use the same two-way line. If this double hunting occurs, one of the offices must retry to hunt other line. This retrying causes such disadvantages as an increase in the number of processes a controller must carry out, and a delay in the processes. A managing system for reducing the double hunting is therefore needed.
(2) Description of the Related Art
An example of a prior art is disclosed in Japanese Patent Publication (Kokai) No. 52-133711 published on Nov. 9, 1977, entitled "Both-way Circuit Operating System". In this reference, two-way lines are connected between two exchange switches, and controllers for these exchange switches assign hunting priorities to the two-way lines such that the priorities for one office are reverse to those for the other office, thereby reducing the probability of occurrence of double hunting.
Another example of a prior art is disclosed in Japanese Patent Publication (Kokai) No. 56-117466, published on Sept. 14, 1981. In this reference, the two-way line managing system classifies two-way lines into a high-priority group and a low-priority group. The two-way lines of the high-priority group in one office are put in the low-priority group in the other office, and the two-way lines of the low-priority group in the one office are put in the high-priority group in the other office. This sort of grouping of the two-way lines may be done according to even and odd numbers thereof.
According to a second conventional system, a two-way line which has been free for a longest time period is selected from the high-priority group. If no free circuit is found in the high-priority group, a circuit which has been freed most recently is selected from the low-priority group. The second prior art system can also reverse the selection priorities of the circuits in both the offices, thereby reducing the probability of an occurrence of double hunting.
The first prior art system explained above has the following problems:
(1) The priorities of circuits for one office are predetermined according to the conditions of the other office. Namely, the one office cannot determine the priorities according to its own conditions. In this way, determining the priorities entails many restrictions.
(2) If there is an inconsistency between both the offices in assigning the priorities, the probability of an occurrence of double hunting may increase.
(3) Both the offices must adopt the same managing system.
(4) The first conventional system is difficult to effectively realize when the two-way lines are controlled by a multiprocessor system as will be later described in more detail with reference to the drawings.
The second prior art system has the following problems:
(1) Similar to the first conventional system, one office must employ the same managing system as that employed by the other office. If the offices employ the first and second conventional systems, respectively, the probability of an occurrence of the double hunting may not be decreased.
(2) If the other office is allowed to employ only the first conventional system, the one office cannot adopt the second conventional system.
The most serious problems are that selection of the first and second conventional systems is determined by a switching machine, and that some switching machines are allowed to employ only one of the systems. If one switching machine is allowed to employ only the first conventional system and the other switching machine only the second system, these switching machines cannot be connected to each other.